1. Field of the Invention
The present invention is directed to an improved internal combustion engine for increasing fuel efficiency while reducing exhaust emissions and a method of operating such an engine. In particular, the present invention is directed to such an engine operable in a premixed charge compression ignition mode.
2. Description of Related Art
Relatively recently, because of the increased regulatory pressure for fuel efficient and low emissions engines, some engine designers have directed their efforts to one type of an internal combustion engine which utilizes premixed charge compression ignition (PCCI). Researchers have used various other names in referencing PCCI combustion including homogeneous charge compression ignition (HCCI) as well as others such as xe2x80x9cATACxe2x80x9d which stands for xe2x80x9cActive Thermo-Atmosphere Combustion.xe2x80x9d (SAE Technical Paper No. 790501, Feb. 26-Mar. 2, 1979), xe2x80x9cTSxe2x80x9d which stands for xe2x80x9cToyota-Sokenxe2x80x9d (SAE Technical Paper No. 790840, Sep. 10-13, 1979), and xe2x80x9cCIHCxe2x80x9d which stands for xe2x80x9ccompression-ignited homogeneous chargexe2x80x9d (SAE Paper No. 830264, 1983). All of these terms are hereinafter collectively referred to as PCCI.
Generally, conventional internal combustion engines are either a diesel or a spark ignited engine, the diesel engine controlling the start of combustion (SOC) by the timing of fuel injection while a spark ignited engine controls the SOC by the spark timing. Initially, it should be understood that SOC refers to the point in time at which a charge within the cylinder begins to ignite. The major advantage that a spark ignited natural gas or gasoline engine has over a diesel engine is its ability to achieve extremely low NOx and particulate emissions levels. The major advantage that diesel engines have over premixed charge spark ignited engines is in its higher thermal efficiency. One key reason for the higher efficiency of diesel engines is its ability to use higher compression ratios than premixed charge spark ignited engines since the compression ratio in premixed charge spark ignited engine must be kept relatively low to avoid knock.
A second key reason for the higher efficiency of diesel engines lies in the ability to control the diesel engine""s power output without a throttle. This eliminates the throttling losses of premixed charge spark ignited engines and results in significantly higher efficiency at part load for diesel engines. Typical diesel engines, however, cannot achieve the very low NOx and particulate emissions levels which are possible with premixed charge spark ignited engines. Due to the mixing controlled nature of diesel combustion, a large fraction of the fuel exists at a very fuel rich equivalence ratio which is known to lead to particulate emissions. Premixed charge spark ignited engines, on the other hand, have nearly homogeneous air fuel mixtures which tend to be either lean or close to stoichiometric, resulting in very low particulate emissions. Another consideration is that the mixing controlled combustion in diesel engines occurs when the fuel and air exist at a near stoichiometric equivalence ratio which leads to high temperatures. The high temperatures, in turn, cause high NOx emissions. Premixed charge spark ignited engines, on the other hand, either have much lower NOx emissions or their NOx emissions can be reduced to very low levels with a three-way catalyst.
Another type of engine that has been recent focus of research and has been proposed and studied in the prior art is a direct injection natural gas engine that utilizes compression ignition. In such an engine, highly pressurized natural gas is injected directly into the combustion chamber during or after compression so that the heat generated by compression ignites the injected natural gas in a manner similar to that of diesel injection applications. Such combustion is typically aided by a glow plug and/or a pilot injection and the direct injection natural gas engine allows higher compression ratios than a comparable spark ignition natural gas engine. Hence, the gross thermal efficiency of a direct injection natural gas engine is known to be higher than that of a spark ignition natural gas engine. However, a direct injection natural gas engine requires the natural gas to be compressed to very high pressures such as 3000 psi or greater which is very difficult to attain. This required compression process requires a substantial amount of work which reduces the brake thermal efficiency of the direct injection natural gas engine. Consequently, whereas the emission performance in a direct injection natural gas engine has been found to be better than a conventional diesel engine, the higher emissions (as compared to a spark ignited engine) as well as complexity and high cost has minimized the commercial appeal.
Unlike the above described internal combustion engines, engines operating on PCCI principles rely on autoignition of a relatively well premixed fuel/air mixture to initiate combustion. More specifically, in PCCI engines, the fuel and air are mixed in the intake port or in the cylinder, long before ignition occurs. The extent of the homogeneity of the mixture may be varied depending on the combustion characteristics desired. Some engines may be designed and/or operated to ensure that the fuel and air are mixed into a homogeneous, or nearly homogeneous, state. Also, an engine may be specifically designed and/or operated to create a somewhat less homogeneous charge having a small degree of stratification. In both instances, the mixture exists in a premixed state well before ignition occurs and is compressed until the mixture autoignites. Thus, PCCI combustion event is characterized in that: 1) the majority of the fuel is sufficiently premixed with the air to form a combustible mixture throughout the charge at the time of ignition; and 2) ignition is initiated by compression ignition. In addition, PCCI combustion is also preferably characterized in that most of the mixture is significantly leaner than stoichiometric to advantageously reduce emissions, unlike the typical diesel engine cycle in which a large portion, or all, of the mixture exists in a rich state during combustion. Because an engine operating on PCCI combustion principles has the potential for providing the excellent fuel economy of the diesel engine while providing NOx and particulate emissions levels that are much lower than that of current spark ignited engine, it has also recently been the subject of extensive research and development.
It is now known that for efficient, low emission PCCI combustion, it is important to have the combustion event occur at an appropriate crank angle during the engine cycle. In this regard, it has further been found that the timing of the start of combustion (SOC) and the combustion rate (therefore combustion duration) in a PCCI engine primarily depend on various combustion history values such as the temperature history; the pressure history; fuel autoignition properties (e.g. octane/methane rating or activation energy); and trapped cylinder charge air composition (oxygen content, EGR, humidity, equivalence ratio etc.). However, it should be understood that the term PCCI does not exclude the use of ignition timing mechanisms such as pilot injections and spark ignition known in the art that are used to precisely time the ignition of the premixed charge. Whereas the premixed charge may combust due to compression, such ignition timing mechanisms aid in initiating the SOC of the premixed charge at a precise time to ensure desirable combustion characteristics. This is in contrast to non-PCCI engines such as conventional gasoline engines with spark ignition in which the premixed charge of gasoline and air would not ignite at all without the spark.
A premixed charge compression ignition engine with optimal combustion control with various control features for controlling SOC and the combustion rate is disclosed in the patent application Ser. No. 08/916,437 filed on Aug. 22, 1997, currently assigned to the Assignee of the present invention. This application has also been published as International Patent Application No. PCT/US97/14815. As disclosed in the ""437 application, active control is desirable to maintain the SOC and duration of combustion at the desired location of the crankshaft and at the desired duration, respectively, to achieve effective, efficient PCCI combustion with high efficiency and low NOx emissions. In this regard, the ""437 application discloses a PCCI engine comprising a combustion history control system that includes at least one of a temperature control system for varying the temperature of the mixture of fuel and air, a pressure control system for varying the pressure of the mixture, an equivalence ratio control system for varying an equivalence ratio of the mixture and a mixture autoignition property control system for varying an autoignition property of the mixture. The engine uses an operating condition detecting device that detects an engine operating condition and provides a corresponding signal to a processor that generates one or more control signals to control the combustion history control system such as the temperature control system, the pressure control system, the equivalence ratio control system and/or the mixture autoignition property control system. In this manner, the variable control the combustion history of future combustion events may be attained. A start of combustion (SOC) sensor such as a cylinder pressure sensor may be used to sense the start of combustion so that effective feedback control may be also attained.
The ""437 application further discloses the use of an injector to inject additional gas or liquid such as diesel fuel into the cylinder to time the PCCI combustion event. In this regard, diesel fuel may be injected either early in the compression event or later in the compression event near top dead center (TDC) to initiate a PCCI combustion event. Thus, the late injection serves a similar function to diesel pilot operation in that it adds a small amount of stratified fuel which can be either spark or compression ignited to help ignite the premixed fuel to thereby initiate the PCCI combustion event. Such diesel pilot has been found to be advantageous in that it provides an effective way to initiate ignition of the premixed charge and to control SOC in a PCCI engine. As further discussed, such pilot has also been effective to ensure PCCI combustion especially during the transition between the different modes of operation or during engine starting. Thus, in accordance with the ""437 application as well as the know prior art techniques of diesel pilot operation, the late injection occurs prior to the PCCI combustion of the premixed charge and serves the function of ensuring that the premixed charges is ignited to initiate the PCCI combustion event.
Despite these significant recent developments in the technology of internal combustion engines, there still exists an unfulfilled need to further increase the fuel efficiency of internal combustion engines while minimizing exhaust emissions. These further improvements to exhaust emissions are desirable and necessary to ensure meeting the ever increasingly stringent government emissions requirements, especially with respect to NOx emission levels which has been difficult to reduce further using the presently known technology and methods.
In view of the foregoing, it is an object of the present invention to provide an improved internal combustion engine with increased fuel efficiency and a method for operating such an engine.
Another object of the present invention is to provide such an engine and method that reduces exhaust emissions.
A third object of the present invention is to provide an internal combustion engine and method that operates in a premixed charge compression ignition mode with increased fuel efficiency and reduced emissions.
Yet another object of the present invention is to provide such an internal combustion engine and method that increases fuel efficiency and reduces emissions by providing a post-ignition injection after the initiation of ignition of the premixed charge.
Still another object of the present invention is to increase brake mean effective pressure (BMEP) while maintaining the peak cylinder pressure below a desired maximum cylinder pressure using the post-ignition injection.
In accordance with the preferred embodiments of the present invention, these objects are obtained by an improved internal combustion engine operable in a premixed charge compression ignition mode where at least a portion of fuel is combusted in a PCCI combustion event, comprising an engine body with a piston assembly, a combustion chamber formed in the engine body by the piston assembly, an intake system for delivering intake air to the combustion chamber, a mixing device that mixes a first fuel with the intake air to provide a premixed charge of air and the first fuel, a direct fuel injector adapted to directly inject a second fuel into the combustion chamber, and a control system adapted to control the direct fuel injector in a manner to provide a post-ignition injection where the second fuel is directly injected into the combustion chamber after onset of ignition of the premixed charge in the combustion chamber. In this regard, the second fuel is preferably injected into the combustion chamber at least one of during combustion of the premixed charge and/or shortly after combustion of the premixed charge in the combustion chamber.
In accordance with one embodiment, the first fuel and the second fuel are both the same type of fuel such as natural gas or diesel. In another embodiment, the first fuel and the second fuel are different types of fuels. In a preferred embodiment, the first fuel is natural gas while the second fuel is diesel. In accordance with one preferred embodiment, the control system is further adapted to control at least one of injection timing, injection duration, injection rate and injection amount of second fuel injected by the direct fuel injector. In this regard, the control system may be adapted to variably control amount of the second fuel injected by the direct fuel injector relative to amount of the first fuel based on at least one of an operating condition and operating mode of the internal combustion engine. The second fuel injected by the direct fuel injector may constitute 0.1 to 50 percent of total fuel combusted in the combustion chamber, or preferably, constitute 0.1 to 25 percent of total fuel combusted in the combustion chamber. Moreover, if the internal combustion engine includes a plurality of cylinders, the amount of second fuel injected may be varied in fewer than all of the plurality of cylinders at a time.
In accordance with still other embodiments of the present invention, the internal combustion engine may further include a sensor that generates a signal indicative of cylinder pressure in the combustion chamber. The sensor may be at least one of a pressure sensor, accelerometer, ion probe, optical diagnostic, strain gage, load washer, fast thermocouple, torque sensor, RPM sensor and emissions sensor. The direct fuel injector may be adapted to inject the second fuel when the sensor senses a predetermined reduction in cylinder pressure in the combustion chamber. In this regard, the injection rate of the second fuel may be controlled by the control system to maintain a substantially constant cylinder pressure that is not greater than a desired maximum cylinder pressure during a predetermined range of motion of the piston assembly, while increasing the brake mean effective pressure (BMEP) of the engine.
The mixing device may include at least one of a carburetor, a throttle body injector, and a port fuel injector that is adapted to mix air and the first fuel upstream of the combustion chamber. Alternatively, or in addition, the mixing device may be a fuel injecting device adapted to directly inject the first fuel into the combustion chamber. In such an embodiment, the fuel injecting device may be the direct fuel injector itself so that direct fuel injector provides both the first fuel and the second fuel to the combustion chamber, and may further include a high pressure system for pressurizing the first fuel prior to directly injecting the first fuel into the combustion chamber. In other embodiments, the mixing device may be a first direct fuel injector adapted to directly inject the first fuel into the combustion chamber and the direct fuel injector may be a second direct fuel injector adapted to directly inject the second fuel into the combustion chamber. Moreover, in any of these embodiments, the direct fuel injector may be further adapted to provide an injection such as a pilot injection or early control injection prior to ignition of the premixed charge to time the start of combustion of the premixed charge in the combustion chamber.
In addition, in accordance with the embodiments of the present invention, these objects are obtained by a method of operating an internal combustion engine in a premixed charge compression ignition mode where at least a portion of fuel is combusted in a PCCI combustion event, the internal combustion engine including a piston assembly defining a combustion chamber, the method comprising the steps of delivering at least intake air to the combustion chamber during an intake stroke of the piston assembly, mixing a first fuel with the intake air to provide a premixed charge of air and the first fuel, performing a compression stroke of the piston assembly after the intake stroke, igniting the premixed charge, and directly injecting a second fuel into the combustion chamber after onset of ignition of the premixed charge. In this regard, the second fuel is preferably directly injected into the combustion chamber at least one of during the combustion of the premixed charge and/or shortly after combustion of the premixed charge in the combustion chamber.
In accordance with the other embodiments of the present method, the first fuel and the second fuel may be both same type of fuel such as natural gas. Alternatively, the first fuel and the second fuel may be different types of fuels. In this regard, the first fuel may be natural gas and the second fuel may be diesel. The present method may also include the step of controlling at least one of injection timing, injection duration, injection rate and injection amount of the second fuel based on an operating condition of the internal combustion engine. In this regard, the present method may include the step of variably controlling the amount of second fuel injected relative to amount of the first fuel based on at least one of an operating condition and operating mode of the internal combustion engine. The second fuel directly injected into the combustion chamber may constitute 0.1 to 50 percent of total fuel combusted in the combustion chamber, or preferably, constitute 0.1 to 25 percent of total fuel combusted in the combustion chamber. Moreover, if the internal combustion engine includes a plurality of cylinders, the amount of second fuel injected may be varied in fewer than all of said plurality of cylinders at a time.
In accordance with another embodiment, the present method may also include the step of sensing cylinder pressure in the combustion chamber, and directly injecting the second fuel based on sensed cylinder pressure. In this regard, the cylinder pressure may be sensed by a sensor that generates a signal indicative of cylinder pressure such as at least one of a pressure sensor, accelerometer, ion probe, optical diagnostic, strain gage, load washer, fast thermocouple, torque sensor, RPM sensor and emissions sensor. The second fuel may be directly injected into the combustion chamber upon sensing a predetermined reduction in cylinder pressure in the combustion chamber. Moreover, the present method may also include the step of controlling the injection rate of the second fuel to maintain a substantially constant cylinder pressure in the combustion chamber for a predetermined range of motion of the piston assembly, the substantially constant cylinder pressure being not greater than a desired maximum cylinder pressure in the combustion chamber.
The first fuel may be mixed with the air by at least one of a carburetor, a throttle body injector, a port fuel injector that injects the first fuel into the air upstream of the combustion chamber, and a direct fuel injector that injects the first fuel into the air in the combustion chamber. Moreover, the present method may also include the step of providing an injection such as a pilot injection and/or early control injection prior to ignition of the premixed charge to time the start of combustion of the premixed charge in the combustion chamber.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention when viewed in conjunction with the accompanying drawings.